Talk to an IBCLC before you start supplementing. Do you have a strong letdown? Does your baby spit up a lot? With a strong letdown sometimes the baby has a hard time keeping up and isn't able to take in all the milk and/or spits a lot of it back up. Also, is baby dressed warmly enough? I know a couple whose baby had a hard time gaining weight and it turned out that they just needed to put more clothes on her. She was spending too many calories keeping her body temp up and couldn't gain weight. Once she had more clothes on she was able to gain easily. Just a couple thoughts off the top of my head.

Also, are you and/or the baby's father on the smaller side? Is it possible that your baby's just small? If she's gaining steadily on her own curve, then she should be fine.

Other thoughts: is she swaddled at night? Sometimes babies don't wake to feed at night when swaddled because they can't wave their arms to wake themselves up. Just something else to consider.